The present invention relates to an ISDN communication system using ISDN (Integrated Services Digital Network) for data communication, and in particular to a technique for trouble processing in an ISDN communication system.
As a conventional technique for trouble processing in data communication systems using private lines, there is known a technique such as a method for recovery from trouble in line adaptor as described in JP-A-63-99653. In communication control apparatuses using this technique, one or more stand-by communication adapters are provided for a plurality of active communication adapters. Upon occurrence of a trouble in an active communication adapter, information set in the faulty communication adapter is restored in a stand-by communication adapter and communication processing is reopened. Thereby processing for recovery from trouble is performed while continuing the communication without bringing a line into a down state.
On the other hand, private lines have been used as communication lines in the data communication field. However, it is expected that ISDN (Integrated Services Digital Network) will be hereafter used widely. (In Japan, commercial service was started since 1987.)
The ISDN is a digital public network into which various communication services are integrated. In the same way as public lines such as telephone lines, communication is managed by using the unit referred to as call. At the time of start/end of communication, therefore, a call establishing/releasing procedure must be carried out.
In the above described conventional technique of restoring information, which has been set in a faulty communication adapter, in a stand-by communication adapter, a measure to meet the ISDN has not been considered. Thus this configuration is not suitable for communication systems using ISDN's allowing bus distribution.
That is to say, in the conventional technique described in the aforementioned JP-A-63-99653, lines are independent by taking hardware performing communication control as the unit and lines are switched. Upon occurrence of a line fault, therefore, lines must be switched. Accordingly, synchronous processing is needed, and there is a fear of delay caused by line switching.
Further, when a communication adapter has recovered from a trouble in case this conventional technique is applied to a communication system using ISDN, a new call establishing procedure must be carried out again to establish a call from a stand-by communication adapter.
This fact means use of lines which are physically and logically independent of those used before the occurrence of the trouble. Accordingly, there occurs a problem that the other party of the communication must be requested to perform line switching.
Further, the call used by the faulty communication adapter remains in use until the release procedure is carried out by the network or the other party of communication, resulting in another problem.
Further, if call establishment is performed whenever the communication adapter is switched, rapid switching processing cannot be conducted, resulting in still another problem.